1. Field of the Invention
This invention relates to a magnetic sensor for an encoder which can detect N- and S-poles arranged at a random interval on the drum of the encoder.
Referring now to FIGS. 6 and 7, a conventional type of magnetic type movement detecting device will be described. The conventional type of magnetic rotary encoder which is one example of such a magnetic type movement detecting device as described above is comprised of a rotary drum 3 and a magnetic sensor oppositely arranged against the rotary drum 3 in a non-contacted condition. The magnetic recording medium arranged at a circumferential surface of the rotary drum 3 is provided with several linear-form poles with their N-poles and S-poles alternatively arranged in a particular spacing (a specified magnetizing pitch) which constitutes a scale, crossed at a right angle with a thickness direction of the magnetic recording medium and with a horizontal magnetization in a direction parallel with the relative movement between the rotary drum and the magnetic sensor. From the track which is comprised of a linear arrangement of these magnetic poles, magnetic fluxes are discharged which are directed toward the space in which the magnetic sensor is arranged These magnetic fluxes cross at a right angle to the magnetoresistance effective magnetic thin films 1, 2 (hereinafter referred to as "MR thin film") shown in FIG. 6 which are a pair of magnetic flux detecting portions arranged in the magnetic sensor. The MR thin films 1 and 2 are made of ferromagnetic material such as Fe-Ni and the like, have a narrow width direction which is a direction of relative movement with the tracks wherein a density of crossing magnetic fluxes and their polarities are varied under a rotation of the rotary drum, and have a long direction crossing at a right angle this relative movement direction, and have a strip-like shape with their longitudinal sides being adjacent in parallel to each other. Both ends of MR thin films 1 and 2 are provided with an electrode for use in applying and connecting a bias current. One of the connecting electrodes arranged at MR thin films 1 and 2 is connected by a conductive pattern so as to act as an output terminal for use in providing a detecting signal, and the connecting electrodes of each of the remaining ends are connected to a positive pole and a negative pole of the power supply through a conductive pattern, respectively. In this way, the bias currents are applied oppositely to each other in a longitudinal direction of the strip-like lines to the MR thin films 1 and 2 connected in series with the power supply. An electrical resistance of each of MR thin films 1 and 2 with this bias current applied is such that thin films 1 and 2 each show substantially the same intermediate resistance value, while the magnetic fluxes may not be crossed with these MR thin films 1 and 2. As the density and polarity of the magnetic fluxes crossing MR thin films 1 and 2 are varied, an electrical resistance between the connecting electrodes connected to each of both ends of MR thin films 1 and 2 is varied from a maximum value to a minimum value so as to detect a variation in voltage at a common connection point between MR thin films 1 and 2. As shown in a top plan view in FIG. 6, the arrangement of each of MR thin films 1 and 2 having such a characteristic of a ferromagnetic-electrical conversion as described above is made such that a spacing in the directions of relative movements of the tracks indicated by arrows in FIG. 6 shows a pitch spaced apart by 1/4.lambda. in respect to a spacing .lambda. of magnetic poles having the same polarities shown in FIG. 7(a). A variation of electrical resistance in respect to a direction of the magnetic fluxes crossing each of the MR thin films 1 and 2 arranged with a pitch of 1/4.lambda. and with a power supply voltage of +5 V applied, for example, in series therewith has a shape magnetic anisotropy characteristic in which it is decreased with the magnetic fluxes crossing with each other in the width directions having a narrow strip-like line and it is dependent upon a pattern shape having no variation in resistance with the magnetic fluxes crossed in a longitudinal direction and a film thickness direction. This characteristic of magnetic anisotropy is not only dependent upon a shape, but also similarly applied to the MR thin film having a ferromagnetic thin film formed in the magnetic field applied in advance.
Under such an arrangement, if one MR thin film 1 is opposite just above the magnetic pole indicated by "N" or "S" in FIG. 7(a), the other MR thin film 2 is opposite to a position intermediate between the magnetic poles indicated by "N" and "S" or "S" and "N" in FIG. 7(a). Under this condition, a resistance value of one MR thin film 1 becomes a maximum value and a resistance value of the other MR thin film 2 becomes a minimum value, and then a detected signal appearing between the output terminal and the ground line (a power supply terminal of the negative pole) is outputted with the lowest voltage value. In turn, under a condition in which one MR thin film 1 is opposite to an intermediate position of magnetic poles indicated by "N" and "S" or "S" and "N" in FIG. 7(a), the other MR magnetic pole 2 is opposite to a part just above the magnetic pole indicated by "N" or "S" in FIG. 7(a). Under this condition, a resistance value of one MR thin film 1 becomes a minimum value, a resistance value of the other MR thin film 2 becomes a maximum value, and a detected signal appeared between the output terminal and the ground line (a power supply terminal of a negative pole) is outputted with the highest voltage value. In this way, as a result of a continuous rotation of the rotary drum, the detected signal may repeat a minimum voltage value and a maximum voltage value in response to a repetition of magnetic poles in which "N" and "S" are alternatively varied as shown in FIG. 7(a) in correspondence with a rotational angle of an amount of movement of the rotary drum and its varying speed. If MR thin films 1 and 2 are constructed such that they cross each other at a maximum density of magnetic flux only when they are opposite to an intermediate position of the magnetic poles indicated by "N" and "S" and "S" and "N" in FIG. 7(a), a signal indicated by a sine wave (for example, +2.5 V at the center) in FIG. 7(b) is outputted. Then a relative amount of movement between the rotary drum and the magnetic sensor and their speeds are discriminated by this sine wave. In addition, an output waveform is outputted in a shape of a trapezoid for the waveform caused by this detecting operation in the case that each of MR thin films 1 and 2 is opposite to a position except one just above the magnetic poles indicated by "N" and "N" in FIG. 7(a) and cross each other with a density of magnetic flux at a level where a variation of the resistance value is saturated.
In order to use a rotary encoder as a mode switch or an absolute switch, a magnetic sensor for an encoder which can detect the poles arranged at a random interval in response to the mode on a rotary drum is desired. However, the conventional magnetic sensor for the encoder can merely detect the poles arranged at an equal interval, but cannot detect the poles arranged at random intervals.